A heat radiating member and an electrical junction box are provided that have a simple configuration and are capable of quickly radiating heat generated by a semiconductor device. A support member, which receives heat from a substrate portion having a mounting face on which a semiconductor device is mounted, via an opposing plate portion opposing the mounting face, and radiates the received heat, includes a recessed portion formed in the opposing plate portion, at a position corresponding to the semiconductor device. The recessed portion has a wall thickness that is greater than that of another portion of the opposing plate portion.